US officials have intercepted a letter sent to a member of the US Congress in Washington which tested positive for the deadly poison ricin.
Senator Roger Wicker, a Republican from Mississippi, is reported to have been the intended target.
Wicker and all Senate employees have now been put on high alert.
The suspect letter is said to have been post marked from Memphis Tennessee.
The incident has caused alarm as it comes one day after the Boston Marathon bombings.
Ricin is a lethal poison and can cause death from exposure to just minute amounts.